Sunlight is normally divided into infrared energy, visible light, and ultraviolet light. Infrared energy consists of wavelength above 760 nanometers. Visible light is defined as radiation between 400 and 760 nanometers. Ultraviolet lights consists of radiation below 400 nanometers. Infrared energy is our main source of warmth. Sunlight supplies energy necessary for photosynthesis in the living plants. In fact, it is essential for all living things on earth.
In leisure time, some people like to bask in the sun to get a healthy looking tan. Many people like to enjoy months of uninhibited sunshine by participating in other outdoor activities. However, research has found that increased exposure to ultraviolet rays from the sun causes skin cancer, cataracts in human eyes, sunburn, skin wrinkling, possible immune system damage, and leathery skin. It also causes cacti to shrivel, cattle and sheep to develop conjunctivitis, eucalyptus trees to turn yellow, frog population to decline, and fish population to decrease. Humans are among the living beings most vulnerable to the constant shower of ultraviolet radiation. Unprotected by scales or feathers, we face a rising threat of illness related to sunlight exposure. The new health hazard is challenging our desire to spend time outdoors. The causes of skin cancer have been on the rise steadily for the last 20 years and has become one of the leading causes of death today. One in six Americans might develop skin cancer in their lifetime because sun damage to the skin is cumulative.
Hovering six to 25 miles above the earth, the stratospheric ozone layer is our natural sunscreen, protecting us from some of the sun's harmful rays. Atmospheric scientists at National Aeronautics and Space Administration have used satellites to study the depletion of ozone in the upper atmosphere. There is evidence that a severe depletion of the ozone layer has occurred over the Antarctic, resulting in an ozone hole of about 9 million square miles in 1994 (about 2.5 times the size of the United States of America). The hole in the ozone layer is caused by chlorofluorocarbons (CFCs) which are commonly used as refrigerant and propellant in aerosol sprays. The CFCs undergo a series of chemical reactions in the atmosphere, leading to the production of chlorine monoxide which eats up ozone. Levels of ozone destroying chlorine monoxide in Antarctic are extremely high by August, carving a hole in the ozone shield that filters out damaging ultraviolet radiation in the sunlight. As the wind whips around the globe, the ozone layer becomes thinner at most latitudes. During the last decade, the annual dose of harmful ultraviolet light striking the northern hemisphere rose by about five percent, according to the United Nations Report.
The sun gives off ultraviolet light. The International Commission on Illumination further subdivides the portion of the spectrum into UV-A, UV-B and UV-C rays.
UV-A rays have the longest wavelength of 400 to 315 nanometers and penetrate the skin the deepest. UV-A light is also the most difficult one to screen out, although ozone and clouds do screen some of it out. It goes right through the ozone layer. Many sunscreens do not protect against it very well. It is just as strong at 9 am as at noon.
UV-B rays are in the wavelength region of 315 to 280 nanometers. Although some of them are screened out by ozone and clouds, many of them do reach the earth to harm us. UV-B light is responsible for wrinkling, breaking down the elastic tissue and collagen, and sunburn. UV-B light is probably 100 times more carcinogenic than UV-A light. It causes three types of skin cancer--basal cell cancer, squamous cell cancer, and melanoma. Skin cancers have skyrocketed in the last 20 years, coinciding with our increased outdoor activities and the depletion of the ozone layer. UV-B light is probably the culprit for the tough leathery look of human skin. However, when the ozone layer is thick enough to function properly, it shields us from most UV-B rays.
UV-C rays are the shortest, having a wavelength of 280 to 100 nanometers, and are actually the most dangerous among all ultraviolet rays, but do not reach the earth.
The National Weather Service, urged by the Environmental Protection Agency, the Centers for Disease Control, and the American Cancer Society, has begun a new index of UV radiation on a trial basis to warn people against overexposure to the sun. The index based on atmospheric changes has a scale of from 0 to 15. The higher the number, the higher the risk to skin cancer and the faster that outdoor enthusiast will burn. To predict the Ultraviolet Potential Index, the National Weather Service uses satellites and ground equipment to compute the UV levels through a combination of readings from forecasted cloud cover, temperatures, and local ozone amounts. The more ozone present at a location, the less radiation will reach the earth's surface at that area. A rating of 7 means that fair-skinned people should stay out of the sun or risk high UV exposure.
The new Ultraviolet Potential Index measures potential exposure in five levels as follows, according to the National Weather Service and the American Cancer Society:
0 to 2: Minimal risk of ultraviolet radiation; could be in sun unprotected for more than an hour without skin burning. PA1 3 to 4: Low risk; could be in the sun unprotected for 30 minutes to an hour. PA1 5 to 6: Moderate risk; could be in sun unprotected for 20 to 30 minutes. PA1 7 to 9: High risk of skin damage at 13 minutes. PA1 10 to 15: Very high risk of skin damage occurs at less than 13 minutes.
As of July 1994, only 85 cities in the United States were given the predicted Ultraviolet Potential Index on a trial basis. It would be very difficult to include all parts of the nation. These predicted index numbers can serve as general guidelines only. The local cloud cover might move away or become thick because weather conditions are unpredictable. Thus, there is a need for a device to warn outdoor enthusiasts against overexposure to the sunlight anywhere.
To reduce the ultraviolet radiation exposure, it is advisable to apply sunscreen, having a Sun Protection Factor (SPF) of at least 15 and above, on the skin. The sunscreen contains ultraviolet light absorbers which remove part or most harmful ultraviolet rays, depending upon the amount of the absorbers in the sunscreen. By applying the sunscreen on the skin, one might mistakenly believe that the skin will not be damaged by the ultraviolet light. In fact, the sunscreen does not completely block all UV-A and UV-B rays in the sunlight. UV radiation cannot be seen, felt, or smelled.
Various devices have been proposed for monitoring exposure to ultraviolet radiation, such as those disclosed in U.S. Pat. Nos. 4,985,632; 5,117,116; 4,308,459; 4,130,760; and 3,787,687. However, previous suggestions have various drawbacks including use of cumbersome mechanical devices, use of carcinogenic compounds for indicating exposure by color indication, or the like.